1. Technical Field
This disclosure relates to a regulator and, more particularly, to a shunt regulator having an over-voltage protection function and a semiconductor device including the shunt regulator.
2. Discussion of Related Art
A regulator is a circuit block that supplies an output voltage having a substantially constant magnitude, even though the magnitude of an input voltage changes. A shunt regulator is a regulator that includes a current shunt to maintain a constant output voltage.
A conventional shunt regulator is shown in FIG. 1 and typically includes an operational amplifier 11 and a PMOS transistor MP1 and supplies a constant supply voltage VDD to a load 13.
The shunt regulator receives a DC input voltage VIN through a resistor R1 and generates a stabilized supply voltage VDD. The operational amplifier 11 receives a reference voltage VREF1 and a voltage from a feedback circuit formed of resistors R2 and R3 and generates an output voltage that changes in response to the fed-back supply voltage VDD. The output voltage of the operational amplifier 11 is stabilized by a capacitor C1 connected to ground GND. The PMOS transistor MP1 forms a current shunt between the supply voltage VDD and the ground voltage GND. The current flowing through the PMOS transistor MP1 increases when the supply voltage VDD increases, and the current flowing through the PMOS transistor MP1 decreases when the supply voltage VDD decreases. Therefore, the supply voltage VDD to the load 13 may be maintained at a substantially constant value.
When the DC input voltage VIN that is inputted to the shunt regulator excessively increases, a large current has to flow through the PMOS transistor MP1. Assuming that the threshold voltage of the PMOS transistor MP1 is VTH, and the gate-source voltage of the PMOS transistor MP1 is VGS, the overdrive voltage of the PMOS transistor MP1 may be expressed as VGS-VTH. VGS-VTH may be a relatively small value, however, because the output voltage of the operational amplifier 11 has a level of about VDD/2. Therefore, the size of the PMOS transistor MP1 must be sufficiently large, so that the large current may flow through the PMOS transistor MP1 without damaging it. That is, the ratio width/length of the gate of the PMOS transistor MP1 should be increased. For example, the gate width of the PMOS transistor may be in the thousands of μm. An MOS transistor having a gate width in the size of thousands of μm occupies a relatively large chip area in a semiconductor integrated circuit. Moreover, if the size of the MOS transistor is too large, the impedance of the MOS transistor is too low to be adapted for use in a radio frequency circuit.